The Deus Ex: Human Revolution Director's Cut does seem to be a matter of pride for Eidos Montreal. Director's Cut builds are a common way to offer new players a jumping-on point long after launch, but few redesign sections of the original game to account for fan feedback. When it comes to Deus Ex: Human Revolution's boss fights, the negative feedback was loud and unanimous. They've been reworked with new level geometry to allow for multiple approaches, thank goodness.
The Director's Cut also includes a 45 minute "making of" video, developer commentary, a strategy guide, and "major enhancements" have apparently been made to the energy system and enemy AI. There have also been "striking visual improvements", which seem to amount to toning down HR's yellow tint, colloquially referred to as "the piss filter" in certain corners of the Internet.
There's a sensible tiered pricing system to the Director's Cut. It's £12 / $20 if you buy it new, and that includes all Tong's mission (originally a pre-order bonus) and the excellent Missing Link DLC. If you own HR without the DLC, you can upgrade to the Director's Cut for £6 / $10. If you own the DLC and want the new boss fights, commentary and other improvements, it'll cost £3 / $5. It's available on Steam now.